Cache is a special memory subsystem that stores frequently used data to facilitate quick access of the CPU to the data. Therefore, cache guarantees the running speed of the computer. A Dynamic Random Access Memory (DRAM) cache improves computer performance by transparently storing data in the DRAM, so that requests for the data can be handled quickly, normally about 60 μs. However, the cost of the DRAM cache is very expensive, and there is hardware limitation to the DRAM cache's capacity, such as memory slot limitation and battery hold up time limitation.
The Solid-State Disk (SSD) cache improves computer performance by transparently storing data in SSD, so that requests for that the data can be handled relatively fast, normally less than 1 ms. Compared with the DRAM cache, the cost of the SSD cache is lower and its capacity is higher. However, there exists a problem concerning waste of SSD disks in the existing SSD cache solution, which increases user cost to a certain extent.